The present invention relates to passive heat transfer devices and more particularly relates to heat pipes utilizing the high latent heat of evaporation and condensation, together with the phenomenon of capillary pumping of a wick, to transfer very high heat fluxes without the addition of external energy.
So-called heat pipes are well known, and typically comprise a condenser and an evaporator connected to one another as a closed system. Referring to FIG. 1, the typical heat pipe 6 comprises an enclosed tube 8 having one end forming an evaporator portion 10 and having another, somewhat-cooler and lower-pressure end forming a condenser portion 12. A wick 14 extends through the heat pipe from the evaporator portion 10 to the condenser portion 12. The surrounding environment is cooled by the evaporator portion and reheated by the condenser portion with the help of fins 15.
In use, liquid refrigerant 11 present in the evaporator portion 10 is heated by the environment, vaporized, and rises into the condenser portion 12. In the condenser portion 12, the refrigerant is cooled by the environment, is condensed with the release of latent heat, and is then pumped back to the evaporator portion 10 by the action of the capillary structure of the material forming the wick 14. The cycle then repeats itself, resulting in a continuous cycle in which heat is absorbed from the environment by the evaporator and released by the condenser.
As illustrated in FIG. 2, it is also known to increase the capacity of heat pipes by incorporating several individual heat pipes 20 in a single assembly 21. Each individual heat pipe is constructed and operable as the heat pipe illustrated in FIG. 1. While such an assembly has a significantly higher capacity than a single heat pipe, it is difficult and expensive to fabricate since each pipe must be individually charged with the proper amount of refrigerant.
Referring now to FIGS. 3A and 6A, it has been proposed to reduce the fabrication and installation costs of heat pipes by utilizing U-shaped heat pipes connected to form serpentine heat pipes. Fabrication costs are decreased through the use of the U-shaped tubes. However, it was thought that the individual tubes of such heat pipes could not be charged with refrigerant and that the serpentine coils would inhibit fluid movement through the heat pipes, thus decreasing their efficiency. One way that such serpentine heat exchangers are rendered useful as heat pipes is to vertically orient a heat exchanger such that the tops of individual coils act as condensers and the bottoms act as evaporators. The individual coils are manifolded together to provide what were thought to be the interconnections required to enable charging of the individual heat pipes. Thus, referring to FIG. 3A, the ends of the individual U-tubes 30A of a heat pipe are manifolded in such a way that the liquid refrigerant can move freely from tube to tube, thus assuring that the liquid level 34A is the same in all tubes. More specifically, the bottoms 35A of the U tubes 30A are pierced and small copper tubes 36A are soldered to the perforations to interconnect the U tubes at their lower ends. The open ends of the adjacent U tubes are manifolded to one another by a straight pipe 37A. The resulting connection allows unrestricted communication between the ends of adjacent tubes and assures that the liquid level is the same in all tubes. Microgrooves 33 are formed in each tube 30A, and the individual tubes are imbedded in aluminum fins 32 to form a heat pipe heat exchanger.
In another configuration utilizing serpentine heat exchangers, two horizontal heat exchangers may be connected to one another such that the lower of the two horizontal serpentine heat exchangers acts as an evaporator and the higher one acts as a condenser. Referring to FIG. 6A, it was thought necessary to manifold the U tubes 60A of the lower section by a first copper tube 63A and to manifold the U tubes 61A of the upper section in the same manner by a second copper tube 64A. The upper ends of these manifolded tubes are connected by a first copper connection tube 62A which serves as a vapor line, while the lower ends of these tubes are connected by a second copper connection tube 65A serving as a return line.
Each of the devices illustrated in FIGS. 3A and 6A works well. However, both devices are expensive to fabricate and to install, thus rendering them unsuitable for many applications.
Moreover, each of these embodiments works on the basic principle of gravity. That being when the refrigerant is condensed in the condenser it returns to the evaporator as a liquid by gravitational force. The gravitational effect occurs in these systems because of the orientation of the condenser related to the evaporator. In order for these arrangements to operate effectively, the condenser must be higher, relative to a ground position, than the evaporator. Thus, if either of these arrangements is installed in a different orientation than as described the devices will not operate properly.
It is also known to use heat pipes to increase the dehumidification capacity or efficiency of an air conditioning system. One such system is described in U.S. Pat. No. 4,607,498, which issued to Khanh Dinh on Aug. 26, 1986. Referring to FIG. 13, this type of air conditioning system 110 includes a primary evaporator 124 and a heat pipe heat exchanger 126 which is provided to increase the dehumidification capacity of the system during cool and humid hours. This heat pipe consists of a pair of manifolded heat exchangers of the type illustrated in FIG. 6A. A first heat exchanger 128 serves as an evaporator and is located between an inlet of the air conditioner and the primary coil 124. A second manifolded heat exchanger 130 is located between the primary evaporator 124 and the outlet of the housing and serves as a condenser of the heat pipe. The heat sections 128 and 130 are interconnected by a vapor line 134 and a return line 140.
The heat pipe heat exchanger 126 operates as follows:
Warm air enters the housing from the inlet and is cooled slightly as it passes over evaporator 128, thereby vaporizing the liquified refrigerant present in the evaporator. The air then passes over the primary evaporator 124, where it is cooled further. Meanwhile, the vaporized refrigerant rises out of the header of the evaporator 128, through conduit 134, and into the header of condenser 130. The refrigerant in the condenser 130 is cooled by air exiting the primary evaporator 124 so that it is liquefied while simultaneously reheating the air. The liquified refrigerant then flows downwardly into the inlet of evaporator 128 via conduit 140, and the process is repeated.
While the heat pipes described above significantly improve the efficiency of air conditioners, the manifolded heat pipes require additional machining of the serpentine coils and require that headers be connected to the ends of the coils. Accordingly, they are relatively difficult and expensive to fabricate. Thus, the cost of such heat pipes may render impractical their use in many applications, including many conventional air conditioning systems.